The overall objectives of the total project are: (1) to better understand the pathogenesis and biologic behavior of pituitary adenomas and (2) to correlate their histologic, immunocytologic and electron microscopic features with the clinical and biochemical findings. The approach aims to provide a morphologic classification which could be useful to neurosurgeons, radiologists as well as endocrinologists, and it would permit one to make predictions from the morphologic features to growth rate and the therapeutic responsiveness.